1. Field of the Invention
The present invention relates to a compound semiconductor substrate and a device to which the compound semiconductor substrate is applied.
2. Description of the Related Art
A transistor having a hetero junction of compound semiconductors so as to generate two-dimensional electron gas is known as “high-electron-mobility transistor”, or its shortened form “HEMT”. Because of its high electron mobility, HEMTs are widely used for producing high-speed switching devices. As formation of the hetero junction in general requires epitaxy of good quality, single-crystalline GaN or sapphire is in general used as a substrate.
Recently, application of Si and SiC has been urgently studied because of its better thermal conductivity, better availability and reasonable costs. However, Si and SiC considerably differ from the compound semiconductors in lattice constant. Any compound semiconductors epitaxially grown on Si or SiC have lattice mismatching and therefore cause strain in the structure, which affects electric properties of the transistor.
Interposition of any buffer structure to ease lattice mismatching may prevent generation of strain. Some studies in this way have achieved a level of success in producing HEMT structures on Si or SiC substrates. However, thus produced structures still fail to perform satisfactory electric properties particularly in view of a switching speed and a high-frequency property.